The present embodiments relate to calibration for functional imaging. Calibration is provided for quantitative functional imaging.
Functional imaging uses a radioisotope or radiotracer to determine metabolic function within a patient. For example, the uptake of the radiotracer by tissues in the body is measured. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are two types of functional imaging. The emissions from the radiotracer are detected in the functional imaging. The activity concentration (i.e., the concentration of the radiotracer from different locations) is reconstructed from the detected emissions.
The reconstruction uses the sensitivity of the detector for the emissions. This sensitivity may be calibrated, but contributes a possible source of error in quantitative functional imaging. If a class standard sensitivity is used, the detector specific sensitivity may be different. Similarly, the dose applied to the patient introduces another source of error in quantitative functional imaging. The dose value for the liquid isotope applied to the patient may be inaccurate.
For quantitative functional imaging, both accurate activity concentration and uptake values are desired. The goal is to provide a global baseline that is free of system (detector and dose calibrator) variability so that any measured change for a patient over time in either quantity is due to metabolic reasons. Clinically useable cross-calibration accounting for detector sensitivity and dose sensitivity in SPECT has been problematic because the incompatibility of isotropic point sources made with liquid radiotracers and anisotropic factory-calibrated sealed sources with long-lived isotopes. Scaling the class standard gamma camera planar sensitivity for emission energy of a given radiotracer with the system specific sensitivity measured using factory calibrated sealed point sources having emission energies close to the emission energies of the radiotracer for the patient may produce activity concentrations that are accurate at the expense of inaccurate uptake values. On the other hand, scaling the class standard gamma camera planar sensitivity using system specific sensitivity measured with a locally made point source of the radiotracer for the patient may produce accurate uptake values but inaccurate activity concentrations.